Because of their strength, heat resistance, and chemical resistance, polyester containers, films, and fibers are an integral component in numerous consumer products manufactured worldwide. In this regard, most commercial polyester used for containers, films, and fibers is polyethylene terephthalate polyester (or PET).
Polyester resins, especially polyethylene terephthalate and its copolyesters, are also widely used to produce rigid packaging, such as food and beverage containers. Polyester containers produced by stretch-blow molding possess outstanding strength and shatter resistance, and have excellent gas barrier and organoleptic properties. Consequently, such light-weight plastics have virtually replaced glass in packaging numerous consumer products (e.g., carbonated soft drinks, water, fruit juices, and peanut butter).
In conventional processes for making polyester container resins, modified polyethylene terephthalate resin is polymerized in the melt phase to an intrinsic viscosity of about 0.6 deciliters per gram (dl/g), whereupon it is further polymerized in the solid phase to achieve a higher intrinsic viscosity that is better suited to container formation. Thereafter, the polyethylene terephthalate may be injection molded into preforms, which in turn may be stretch-blow molded into bottles or other containers.
To achieve fast production rates in the stretch-blow molding process, the preforms are heated in an infrared oven. The use of additives that absorb infrared radiation speed the heating of the preforms allowing for faster production rates. Unfortunately, these additives reduce the L* color of the preforms, causing them to appear darker. Reheat additives can also make the preforms appear cloudier or hazier, which is not desired in the industry.
Therefore, there is a need for polyethylene terephthalate resin containing a reheat additive that can maintain a high L* color value in preforms while maintaining good clarity and a fast reheat.
The production of preforms to be stretch-blow molded into bottles or containers benefits from the use of an infrared absorbing additive to improve the cycle time in the manufacturing process. The prior art for reheat additives includes carbon blacks with a particle size between 10 and 500 nanometers (Pengilly: U.S. Pat. Nos. 4,408,400; 4,476,272, 4,535,118), metallic antimony particles from residual catalyst (Tindale: U.S. Pat. Nos. 5,149,936 and 5,529,744), and others well known in the art. These include, but are not limited to, black iron oxide, iron phosphide, copper chromite spinel, and titanium nitride. Each of the reheat additives absorbs infrared radiation to improve the heating rate of preforms in the stretch blow molding process. However, each additive, to some degree, reduces the L* color value of the preform making the preform darker. These additives also increase the haze in the preform making the preforms cloudier.
The Pengilly patents (U.S. Pat. Nos. 4,408,400; 4,476,272; and 4,535,118) state that the preferred mode of the invention is to use either a furnace or channel carbon black with a primary particle size of 15 to 30 nanometers. The preferred additive concentration has been given as 1.5 to 3.5 parts by weight per million parts by weight of polyester resin.
The different types of carbon blacks are not referenced in the Pengilly patents. There are many different types of carbons, each with specific ranges of particle sizes and characteristics. Several common carbon black types include furnace, thermal, channel, lamp black, and bone carbon black.
Harrison et al. (U.S. Pat. No. 7,816,436) describe the use of thermal or furnace carbon blacks in PET and PP preforms, wherein the preferred particle size of the carbon black particles is in a range of 200 to 500 nanometers, preferably 250 to 300 nm, in an amount of 3 to 50 ppm, to improve reheat performance. They do not discuss other types of carbon black materials.
Of the above carbon black materials, the furnace carbon black is by far the most common and widely manufactured. The furnace and channel carbon blacks are materials with a primary particle size range from 5 to 100 nanometers. The process typically uses aromatic oils as feedstock. The thermal carbon blacks have much larger particle sizes between roughly 250 and 340 nanometers. The thermal carbon blacks are made from natural gas by cracking away hydrogen against heated refractory bricks in a dual reactor system. Lamp black carbon black forms a distinct species of carbon black with a primary particle size from 100 to 160 nanometers. The lamp black carbon black is typically produced by burning high purity waxes and/or oils and collecting the soot. The lamp black process is one of the oldest processes known for forming carbon black.
We have found that an optimum exists for maximizing preform reheat temperature while maintaining good preform clarity and L* color value. Surprisingly, the carbon blacks which an average particle size in a range of from 100 to 160 nanometers, particularly lamp black carbon blacks, provide a faster reheat than the furnace carbon blacks, but show an improvement in haze at equivalent reheat temperatures compared to the thermal carbon blacks. Particularly, this invention has shown that a lamp black carbon black with a particle size range between 100 and 160 nanometers yields a fast reheat rate with excellent L* color and clarity.